Crop Sequence Effects On Wheat Water Use, Productivity and Economic Returns.

Intensified crop sequences reduce the fallow fraction of semi-arid cropping systems and increase the fraction of precipitation available during the cropped period; however, reduced opportunity for soil water recharge can limit crop water use and productivity. A long-term crop sequence study, based on winter wheat (phase I), was established to evaluate cropping intensity effects on crop precipitation use, biomass productivity, canopy and yield formation, and net economic returns in the central U.S. High Plains. Cropping intensity effects were represented by two three-year crop sequences including corn or grain sorghum (phase II) and fallow (phase III) and six three-year continuous crop sequences including the feed-grains (phase II) and spring canola, soybean or sunflower (phase III). Crop water use was determined for growth-development intervals which were bounded by the stages emergence, spring green-up, anthesis and physiological maturity. Canopy formation was quantified at anthesis; biomass and grain yield production were quantified at maturity. Drought growing conditions persisted for four of the seven years. Results are presented for the wheat phase. Continuous cropping reduced water available to wheat in fall, spring and grain-fill intervals, relative to wheat following fallow; greater soil water depletion following sunflower, relative to canola, was augmented in dry years relative to wet years. Water use during spring and grain fill periods was greater with grain sorghum as feed-grain, relative to that with corn. In comparison to wheat after fallow, continuous cropping reduced canopy formation, biomass productivity, components of grain yield and harvest index. Also, wheat following canola performed slightly better than wheat following soybean or sunflower. Analysis of covariance indicated water use accounted for a portion of cropping intensity effects. Average annual net returns for the wheat phase were least for wheat grown in continuous crop.